Chloroorganosilicon compounds have long been articles of commerce. Such materials are useful in a variety of known applications or as intermediates in the preparation of other organofunctional silicon compounds. Such other organofunctional silicon compounds are prepared by replacing chlorine atoms, in chloroorganic groups attached to silicon atoms in such compounds, with functional groups including those derived from ammonia and its organic derivatives, hydrogen sulfide and its organic derivatives, oligomers of sulfur, carboxylic acid groups including methacrylate groups, and substituted phenoxide groups, among others. The versatility of chloroorganosilicon compounds as intermediates in preparing such organofunctional silicon compounds has contributed to their commercial success in spite of major recognized deficiencies associated with the processes for preparing such chloroorganosilicon compounds.
Thus, since their commercial introduction, there has been an ongoing need in the art to prepare such chloroorganosilicon compounds safely in high yields and efficiencies, with reduced raw material costs, including catalyst costs, and with reduced generation of hard-to-dispose waste by-products.
Current large scale commercial production of such chloroorganosilicon compounds is based on a two-step procedure. In the first step, platinum is used to catalyze a reaction between a hydrosilicon compound, specifically a hydrochlorosilane, and an olefinic chloroorganic compound, specifically allyl chloride or methallyl chloride. This hydrosilation reaction yields a chloroorganochlorosilane, which then must be converted in a second step to the corresponding chloroorganoalkoxysilane or chloroorganosiloxane by methods known in the art.
From a commercial perspective, these hydrosilation reactions suffer from several recognized deficiencies. First, yields in the platinum-catalyzed hydrosilation of allyl chloride, calculated on a molar basis from the limiting reactant, do not exceed 83%. Second, platinum-containing hydrosilation catalysts are very expensive. Third, while the platinum-catalyzed hydrosilation of methallyl chloride with a hydrochlorosilane occurs in higher yield than with allyl chloride, the higher cost of methallyl chloride both on a combined per unit weight and molecular weight basis does not justify its use. Fourth, a second process step is necessary to convert the chloroorganochlorosilane to a chloroorganoalkoxysilane for subsequent use. Even though the product of the first step process, e.g., a chloropropylchlorosilane, can be converted in the second step to the corresponding chloropropylalkoxysilane in high yield, the overall yield from the two-step process still suffers from the modest yield in the first step. Also, none of the chlorine present in the hydrochlorosilane raw material ends up in the final product. It must either be recycled to trichlorosilane via its direct synthesis or to methyl chloride for the direct synthesis of methylchlorosilanes, or it must be destroyed as waste, adding to process cost and complexity.
While there have been recent improvements in yields and efficiencies for platinum-catalyzed hydrosilation reactions between allyl chloride and trichlorosilane or methyldichlorosilane, the results remain disappointing. Such improvements have dealt primarily with the use of various forms of reusable platinum catalysts, to reduce catalyst costs, or with venting by-product propylene, to increase conversion of the hydrochlorosilane raw material to the desired product.
Similar yields and efficiencies also have been reported for the iridium complex-catalyzed direct hydrosilation of allyl chloride with trimethoxysilane. For example, a yield of 78.4% of chloropropyltrimethoxysilane on a molar basis has been reported for the equimolar reaction of trimethoxysilane and allyl chloride using an iridium-cyclooctene complex as the hydrosilation catalyst (Tanaka et al, Journal of Molecular Catalysis, 81 (1993): 207-214). While the reported yield suggests that an incremental improvement over state-of-the-art commercial processes is possible, the cost of iridium is higher than that of platinum. Given that the iridium use level was nearly 672 parts per million by weight of total reaction charge, the catalyst cost is much too high for the process to be of much commercial interest at the present time.
The same researchers also report the results of directly hydrosilating allyl chloride with trimethoxysilane in the presence of several ruthenium complexes. A maximum yield of 73.6% of chloropropyltrimethoxysilane is reported in Table 2 for the reaction between trimethoxysilane and allyl chloride at a 4/l molar ratio. Assuming the use of the "typical reaction procedure," the process may have employed a ruthenium carbonyl catalyst in an amount to provide at least about 155 parts per million by weight of ruthenium metal in the reaction mixture. Apparently, the reaction also was conducted in a sealed reactor under autogenous pressure in the presence of at least 13.3% by weight of toluene solvent for 16 hours at 80.degree. C. The conversion of limiting allyl chloride was complete, and thus the yields of by-products such as propyltrimethoxysilane, tetramethoxysilane and hydrogen, were not insignificant. The same reaction, at a 2/1 molar ratio of trimethoxysilane to allyl chloride, in the presence of the same absolute weights of ruthenium catalyst and toluene solvent, provided a 72.2% yield of chloropropyltrimethoxysilane on a molar basis, with similar by-product yields at complete conversion of limiting allyl chloride. At a 1/1 molar ratio, the conversion of allyl chloride was not complete. At this mole ratio, a maximum yield of only 52.4% was obtained after 16 hours at 50.degree. C., while at 80.degree. C. the yield was only 26.4%. (Table 2).
Thus, while equivalent molar yields of the desired chloropropyltrimethoxysilane were obtained at peak operation relative to other prior art methods using platinum and iridium catalysts, the yields per unit volume of equipment were necessarily low, due to the use of solvent and to the use of large excesses of trimethoxysilane.
Marciniec et al, Journal of Organometallic Chemistry, 253 (1983): 349-362, also reports ruthenium compound-catalyzed hydrosilation of C.dbd.C bonds. In the case of hydroalkoxysilanes, the reaction is said to proceed only in the absence of solvent and in the case of certain catalysts, is enhanced by oxygen. The authors had essentially no success, however, hydrosilating substituted olefins, including allyl compounds.
Yields from the above-reported ruthenium-catalyzed hydrosilation reactions between trimethoxysilane and allyl chloride, although modest, are interesting in view of the use of ruthenium compounds to catalyze other reactions from similar reactants. Formation of undesired, unsaturated by-products is known to occur, at times to a significant extent, in ruthenium-catalyzed reactions of certain hydrosilicon compounds with olefins. Such other reactions include dehydrocondensation reactions wherein hydrosilicon compounds and olefins react to form vinylic silicon derivatives, olefin metathesis reactions wherein two olefins react to form two different olefins, olefin reduction, and olefin isomerization. These other reactions are noted on occasion to occur with the total exclusion of hydrosilation. Surprisingly, the formation of such by-products does not occur to any significant extent in the process of the present invention.
In contrast to the above reported hydrosilation reactions of allyl chloride and of the propensity of ruthenium compounds to catalyze a variety of reactions between hydrosilicon compounds and olefins, the process of the present invention provides chloropropylmethoxysilanes in nearly quantitative yields based on limiting allyl chloride when trimethoxysilane is used. The process of the present invention uses a relatively low molar excess of the hydromethoxysilane, and can operate with a much lower level of the ruthenium catalyst, and with shorter reaction times. The process produces low levels of waste by-products, and can substantially eliminate any need for a solvent, while reducing limitations on the equipment which may be used.